by Greg Toppo, USA TODAY

by Greg Toppo, USA TODAY

A computer science professor at Carnegie Mellon University in Pittsburgh and a native of Guatemala, von Ahn learned a few years ago that about 1.2 billion people worldwide are struggling to learn a foreign language, but most can't afford to pay for instruction.

Meanwhile, the Web needs translating. Even popular sites exist largely in English-only versions: Wikipedia's Spanish site, for instance, is only about one-fifth the size of the original.

So von Ahn developed a "massive-scale online collaboration" tool that uses the first problem to solve the second, and vice-versa.

"If we want to translate the Web, we can't do it with 100 people or 1,000 people," he said. "We literally need millions. And if we need millions of people, we can't quite pay them, because there's millions of them. So then the question was, 'How do we get them to translate stuff for free?'"

His free Duolingo app and website - duolingo.com - invite users to learn one of six languages for free. The gamelike interface is ad-free and encourages users to compete with friends. But here's the thing: After about 45 minutes of instruction, the program begins asking users to translate random sentences into their new language. Von Ahn plans to sell the sentences to sites that want their material translated on the Web. He expects that half of the service to be up and running later this year.

Crowdsourced with others' for greater accuracy, the sentences feed von Ahn's giant effort to translate the Web into Spanish, German, French, Italian and Portuguese - and into English. He plans to add other languages as users indicate they can support the effort.

Students' results, he said, are superior to automated translation software and just shy of professional translators' efforts.

The audacious move shouldn't surprise anyone familiar with von Ahn's work. A MacArthur "genius" fellow, in 2000 he helped develop CAPTCHA, the computer program that uses distorted letters and numbers to help websites verify that humans, not computers, are purchasing concert tickets and the like. He flipped the idea a few years later with reCAPTCHA, which uses unreadable text from books that Google is digitally scanning - it bought the technology in 2009. Now when you buy concert tickets, you're typing words that help digitize the world's printed books.

Launched last June after months in beta testing, Duolingo has about 700,000 active users. Only about one in four lives in the USA. The iPhone app was expected to see its 1 millionth download last Friday.

"The magical thing about this is that something comes out of nothing," he said. "You're doing something that you would have done anyway and it just so happens that this actually becomes useful."

And the massive number of users also means that von Ahn can conduct his own language research. For instance, he wanted to know whether it was more effective to teach Spanish adjectives or adverbs first, so he set up a trial with 50,000 users and found that Team Adjective did better.

Veronique Baloup-Kovalenko, a French and Spanish teacher at Convent of the Sacred Heart, a Catholic girls' school in Greenwich, Conn., began using the app with her classes recently and said it's a good supplement to classroom instruction. Because it allows students to work at their own pace, it makes for good practice. "It's brilliant because everybody is getting something out of it," she said. "It's quick, it's fun and it's easy to access. They love it because it's the element of challenge, of competition among themselves."

Michelle Kindt, a French teacher at Hershey (Pa.) Middle School, said the app as its designed won't make her students fluent. "But it will broaden your vocabulary base, and it will probably teach you enough basics that you will be able to order from a restaurant, maybe have a short conversation with someone regarding things you like to do and don't like to do," she said.

However the crowdsourcing turns out, von Ahn said he feels strongly about always keeping Duolingo free for those wanting to learn a new language. "I've made it pretty clear," he said. "If we ever have to start charging the users, I quit."